The fire-fighting systems of industrial factories, e.g., chemical plants, petrochemical plants, petroleum refineries and well-drilling plants, require that a foam extinguisher be mixed and added at a constant proportion to the water of the fire-fighting network, which is supplied by suitable pumps, to obtain a proper foam extinguisher solution. When the solution is delivered, e.g., by means of the spreaders, it generates a foam which extinguishes the flames, while maintaining, under any operating conditions, its fire-extinguishing characteristics.
Many foam-extinguishers are known in the prior art, which are suitable for use in fire extinguishing. They perform to the maximum extent when they are used in the prescribed proportion of fire-fighting water to foam-extinguisher liquid. In the following, such "foam-extinguisher liquid" is also referred to as an "additive" or "concentrate".
When an excessive amount of foam-extinguisher liquid is used, a lower fire-extinguishing quality is obtained, because with increasing proportions, beyond a certain limit, undesirable or negative results are likely to result on the characteristics of the generated foam, e.g., an excessive increase in foam viscosity, hinders the flowing of the foam.
Additionally, further disadvantages exist, such as an increase in the specific cost of the foam-extinguisher per unit of fire-fighting solution, a shorter autonomy of operation of the fire-fighting system, and the need for the operators to take more frequent actions in order to handle and replenish the foam-extinguisher liquid consumed.
These two disadvantages, can be critical in the emergency situation of a fire.
On the other hand, when too low a ratio of foam extinguisher to water is used, the foam that is produced loses its fire extinguishing properties very rapidly.
In any case, the accuracy of the proportioning of the foam-extinguisher additive should be maintained within specific limits of not higher than +20%, or even less, in order to attain the best effect.
In the present fire-fighting systems, the most commonly used foam-extinguishers are used in aqueous solutions at concentrations at 6%, but the most recent additives are designed for use at 3%, or even at 1%. This is to reduce the amount of foam-extinguisher additive required to be kept in storage, or needed to be purchased under emergency situations, while maintaining the autonomy of operation or, on the contrary, to increase the autonomy of operation while maintaining the same volume of stored additive.
Such a problem proves to be very important, because the flowrates required for fire extinguishing are very high, and the amount of foam-extinguisher--even when used at low percentages--are always considerable.
Depending upon the number of fire-fighting nozzles used, the delivered flowrate of the fire-fighting solution varies within a very wide range, and within this range, the precision in additive addition should be maintained, even during an emergency situation such as a fire.
A further requirement concerns the preparation of the solutions. The possibility that the stored amounts of a type of additive--e.g., a foam-extinguisher which is properly to be used at 6% solution may be exhausted. In these circumstances the device should easily adapt in order to conform to the different proportions of water required for a different foam extinguisher.
A further, and extremely important requirement of fire-fighting systems is that the mixing devices must preferably operate in a stand-alone mode--without energy being supplied from the external environment because under emergency conditions, there could be a severe deficiency of available energy.
In the prior art, many devices for continuous mixing were proposed, but, from a general standpoint, these mixing devices use ejectors which are driven by the energy supplied by the pressure of the fire-fighting system water. These ejectors withdraw the foaming additive from the tank by the depressure generated by the ejector.